1. Field of the Invention
The present invention relates to an apparatus for measuring the weight of a vehicle seat including the weight of a passenger sitting thereon, and more particularly to a seat weight measuring apparatus capable of increasing the safety against an abnormal force acting on the seat, or of alleviating a strength requirement for load sensors. The present invention also relates to a seat weight measuring apparatus having advantages in that the dimensional precision requirements for component parts or installing portions of the apparatus are about the same as the current level for other parts around the seat, or in that an overall thickness of the apparatus can be decreased.
2. Description of the Related Art
Automobiles are equipped with seat belts and airbags to secure safety for passengers. In recent years, there is a trend for controlling the operation of such safety devices according to the weight of a passenger for improved performance of seat belts and airbags. For example, the amount of gas to be introduced into the airbag, an airbag inflating speed, or a pre-tension of the seat belt may be adjusted according to the weight of a passenger. For that purpose, some means are needed for measuring the weight of a passenger sitting on the seat. An example of such means includes a proposal (Japanese Patent Application No. 9-156666 filed by the applicant of this invention) which involves arranging load sensors (load cells) at four corners of the seat under seat rails and summing vertical loads acting on the load cells to measure the seat weight including the weight of the passenger.
The load sensors of the seat weight measuring apparatus described above are preferably physically small with a measuring capacity of up to 50 kg. Such load sensors may include: sensors having a strain gauge attached to (or formed on) a sensor plate that flexes when loaded; piezoelectric type sensors; and capacitance type sensors that detect displacements of an elastic member that flexes when loaded.
However, the deflection strokes of the load sensors described above are very small, and a very high dimensional precision of the members around the sensor is required for a normal functioning of the sensor. In addition, special care should be taken during assembly, to prevent giving an unbalanced displacement to a sensor.
From a strength viewpoint, fixing structures between seat connecting mechanisms and seat fixing portions are required to have a breaking load of 2300 kgf at a seat belt anchoring portion. If the load sensor itself is required to have this breaking load, the rigidity and strength of the sensor have to be very high, making it extremely expensive. In addition, the deflection stroke described above tends to become even smaller. Further, the size of the sensor may become too large to be installed between the seat connecting mechanisms and the seat fixing portions (seat brackets of the chassis).